1. Field of the Invention
The present invention relates to a process of growing a conductive layer from a gas phase, particularly to a process of growing a thin film of conductive material such as the wiring and electrodes of semiconductor devices.
2. Description of the Related Art
To advance the refinement of the semiconductor device structure and to provide increased speed and integration, it becomes more and more necessary to establish a technology of forming highly reliable submicron wiring and electrodes.
In the conventional technology, wiring layers of LSIs formed on a Si substrate are typically formed of sputtered Al. The sputtering process, however, has poor step coverage such that disconnection of conductive layers occurs at the side wall or bottom of openings for electric contact, or via-holes, extending through an insulating layer, or that via-holes are incompletely buried or filled with conductive material and unfilled pores remain in the via-holes, degrading the flatness of the conductive layer, and other layers, lying over the insulating layer.
FIG. 1 schematically illustrates a fine via-hole 133 formed through an SiO.sub.2 layer 132 on a Si or metal substrate 131 and buried with a conductive layer 134, typically on Al, by sputtering. The midthickness line drawn of the conductive layer 134 represents a temporary surface profile of the conductive layer 134 during its growth process.
As shown in FIG. 1, it is unavoidable that sputtering occasionally fails to completely fill the fine via-hole 133 and leaves an unfilled core or pore 135. This is particularly the case when a conductive layer is formed in a via-hole having a large aspect ratio, or a depth-to-diameter ratio, which is necessary to achieve an advanced integration of semiconductor devices.
A proposed solution to the above-mentioned drawback in the sputtering process is to bury a via-hole with a CVD-conductive layer having good step coverage. For this purpose, CVD-Al is most preferred because it has the lowest resistivity among various materials used to form a conductive layer and is therefore practically utilized in a selective growth process on a substrate having top surface areas of different materials.
K. Tsubouchi et al., in Appl. Phys. Lett., 57(12), pp. 1221-1223, proposed a selective CVD process. FIGS. 2A to 2D are cross-sectional views showing typical sequential steps of this process to form conductive layers. The reference numeral 51 denotes an Si substrate, 52 an SiO.sub.2 layer, 53 a via-hole, and 54, 55, 56 Al conductive layers.
Step 1 (FIG. 2A)
An SiO.sub.2 insulating layer 52 is formed on a Si substrate 51 typically by thermal oxidation and is subjected to a photolithography processing to open a via-hole 53 extending therethrough to the top surface of the underlying Si substrate 51.
Step 2 (FIG. 2B)
A CVD process is performed by using a source gas of dimethylalumiumhalide (hereinafter abbreviated as DMAH defined by the formula: AlH(CH.sub.3).sub.2), which has the nature of selectivity to selectively grow an Al layer only on the Si substrate 51, and not on the SiO.sub.2 layer 52, i.e., an Al layer 54 is grown only on the surface of the Si substrate 51 exposed at the bottom of the via-hole 53 extending through the SiO.sub.2 layer 52.
Step 3 (FIG. 2C)
The Al layer 54 is grown to a sufficient thickness to completely bury or fill the via-hole 53, so that the top surfaces of the SiO.sub.2 layer 52 and the completed Al layer 54 are in substantially the same plane. A plasma is then generated to electrostatically charge the top surface of the SiO.sub.2 layer 52, thereby facilitating decomposition of the DMAH gas so that a thin Al layer 55 is grown on the SiO.sub.2 layer 52 as well as on the preexistent Al layer 54 formed in the via-hole 53.
Step 4 (FIG. 2D)
A further Al layer 56 is subsequently grown on the thin Al layer 55 lying over the Al layer 54 and the SiO.sub.2 layer 52, to form a continuous conductor or wiring layer consisting of the Al layers 54, 55, and 56, which extends from the top surface of the Si substrate 51 (i.e., the bottom of the via-hole 53) to the top surface of the SiO.sub.2 layer 52, which is in substantially the same plane as the top end of the via-hole 53.
Because the selective CVD process realizes selective growth of the Al layer 54 only in the via-hole 53 by using an Al-source gas having a selectivity to the substrate, it is necessary to use a plasma in order to unselectively grow the Al layer 55 on both of the different substrates of the prior Al layer 54 and the SiO.sub.2 layer 52 simultaneously.
This process thus requires that a CVD growth apparatus be equipped with a plasma generator, which unavoidably complicates the apparatus design.
N. Takeyasu et al., in "Extended Abstracts of the 1993 International Conference on Solid State Device and Materials ", pp. 180-182, proposed an alternative growth process, in which the CVD selective growth is first performed to bury a via-hole extending through an insulating layer with Al, and a sputtering process is then utilized to effect unselective growth, to achieve the complete burying of via-holes and form a flat conductive layer lying over the via-hole and the insulating layer. This process, however, not only further raises the apparatus cost but also complicates the process steps mainly due to inclusion of the wafer transfer between CVD and sputtering apparatuses.
It is true that the CVD process provides good step coverage in or over fine via-holes or realizes selective growth of a conductive layer on a substrate having top surface areas of different materials, but it has a drawback of a slow growth rate of 0.1 .mu.m/min or less, which is at least by one order slower than the growth rate of 1.0 .mu.m/min obtained by the sputtering process and not only results in a small throughput but also necessitates an elongated time of heating a substrate to provide a desired thickness, during which time undesirable chemical reactions between the conductive layer and the underlying layer proceed and vary the material nature of these layers or substantial variation occurs in the concentration of any impurity diffused in a semiconductor substrate in the preceding process steps.
Therefore, it would be most desirable from the viewpoints of the throughput or productivity and of the apparatus cost if selective and unselective growth processes could be effected using a CVD process alone.